Certain sources (eg. reddit, working nomads) have detectable ways of becoming invalid after some time has passed. These sources can be re-checked periodically to know whether the posts are still valid to be shown.
reddit posts, for instance can be deleted. Also hiring posts have some mechanism for changing their flair to 'Filled' (although I'm not sure how detectable that is).
The working nomad page has a header on the page which says that the posting has expired and also sometimes when you click 'Apply' on their page it brings you to a different page which says that the posting has expired. Both of those can be detected automatically (hopefully).
Certain sources (eg. reddit, working nomads) have detectable ways of becoming invalid after some time has passed. These sources can be re-checked periodically to know whether the posts are still valid to be shown.
reddit posts, for instance can be deleted. Also hiring posts have some mechanism for changing their flair to 'Filled' (although I'm not sure how detectable that is).
The working nomad page has a header on the page which says that the posting has expired and also sometimes when you click 'Apply' on their page it brings you to a different page which says that the posting has expired. Both of those can be detected automatically (hopefully).